Semiconductor lasers, such as laser diodes, produce coherent light waves at a wide variety of wavelengths of the electromagnetic spectrum, and are ideal for many applications, such as optical fiber communication and network systems, laser printers, bar code scanners, compact disc players, etc. Laser diodes and other laser semiconductors generally include an active layer or region, using materials such as, for example, gallium arsenide, gallium indium arsenide, gallium nitride, variations of these materials including zinc oxide and/or phosphorous, or another direct bandgap semiconductor material. Because laser diodes also can be directly modulated at high frequencies (e.g., at several GHz), through modulation of their “drive current”, laser diodes are often used in high speed communication and other networking applications, such as for data transmission, voice communication, multimedia applications, and so on.
Laser performance, including the optical output power provided by the laser, however, varies due to a number of factors including, for example, the laser temperature, the age of the semiconductor laser, and process variations in semiconductor laser fabrication. Such performance variations include a change in the laser transfer function, namely, a change in laser output power (“L”) for a given laser diode current (“I”) (forward bias current). These changes in output power and forward bias current characteristics may be represented graphically as: (1) a shift in a graphical slope of the transfer function (laser output power versus forward bias current (L-I slope)) characterizing the semiconductor laser; and (2) a shift in the laser threshold current (i.e., the level of forward bias current at which the semiconductor laser first demonstrates coherent radiation).
As a consequence, such laser performance variations also cause undesirable variations in an “extinction ratio”, which is defined as the ratio of the optical output power resulting from transmission by the semiconductor laser of a signal representing a data logical 1 (or high), at a first (or comparatively higher) power level, to the optical output power resulting from transmission by the semiconductor laser of a signal representing a data logical 0 (or low), at a second (or comparatively lower) power level (i.e., the ratio of the first power level for a logical one to the second power level for a logical zero). Laser performance variations which diminish the extinction ratio, as a consequence, cause undesirable variations in the signal-to-noise ratio (SNR) of the semiconductor laser. For example, as the L-I slope characterizing the semiconductor laser decreases for given forward bias and modulation currents, the difference between optical 1 (high) signal power and the optical 0 (low) signal power decreases, degrading the signal-to-noise ratio.
Providing an appropriate level of forward bias current, to produce correspondingly appropriate laser output power levels, is also required for many laser applications, such as data transmission applications, both initially when a laser system is installed or started, and also when output power levels may change over time for a given forward bias current. In the prior art, such output power control has focused on maintaining average laser output power levels at a certain level by adjusting the forward bias current until the desired average power level is reached. See C. Olgaard, “A Laser Control Chip Combining a Power Regulator and a 622-Mbit/s Modulator,” IEEE Journal of Solid-State Circuits, Vol. 29, No. 8, August 1994, pp. 947-951. Average output power levels, however, vary with the duty cycle of the laser, i.e., vary with the data being transmitted: when comparatively more optical 1 bits (high power) are transmitted, the average power increases, and when comparatively more optical 0 bits (low power) are transmitted, the average power decreases.
To avoid significant average power level fluctuations as a function of duty cycle, such prior art control systems require power averaging over a considerably long period of time. The averaging period is selected to have a sufficient duration such that the average power over that period will be approximately independent of the transmitted data. Such a long averaging period to determine and achieve appropriate output power levels, however, results in considerable delay and slow response on system start up. More particularly, on system start up, such systems are slow to reach appropriate power levels to begin their selected applications, such as for data transmission.
Also to reduce the dependency of average power levels on the duty cycle, other prior art power control systems have focused on setting the actual power levels of either the optical 1 power level or the optical 0 power level. See P. W. Shumate, Jr., F. S. Chen, and P. W. Dorman, “GaAlAs Laser Transmitter for Lightwave Transmission Systems,” Bell System Technical Journal, Vol. 57, No. 6, July-August 1978, pp. 1823-1836; and F. S. Chen “Simultaneous Feedback Control of Bias and Modulation Currents for Injection Lasers,” Electronic Letters, 3rd January 1980, Vol. 16, No. 1, pp. 7-8. These references utilize an analog current subtraction technique to remove the extra current from a photodetector feedback current signal when transmitting an optical 1 compared to an optical 0 to reduce the data pattern dependence on setting the optical 0 and 1 levels. This technique requires each laser driver (the laser diode and photo detector unit) to be individually trimmed to cancel the data dependence; that is, the current levels must be manually adjusted for each and every fabricated laser drive unit. In addition, this data dependence cancellation technique does not compensate for device aging.
In addition to maintaining appropriate or optimal forward bias current levels, in light of laser performance variations, maintaining appropriate signal-to-noise ratios, may be achieved by maintaining a substantially or significantly constant extinction ratio. Prior art attempts to maintain such a constant extinction ratio include use of an open loop adjustment to a laser diode's operating points, based on the laser die temperature and a look-up table. Unfortunately, such an open loop system provided no guarantee that the corresponding adjustments to the operating points were yielding the desired effect of maintaining a constant extinction ratio, and did not account for the effects of laser aging. In addition, in the open loop system, accurate characterization of the laser was required to program the look-up table, process variations were unaccounted for unless each system was programmed independently, and the laser die temperature may not accurately represent the semiconductor laser's operating temperature.
Another prior art attempt to maintain a constant extinction ratio superimposed a low frequency modulation to the transmit power level associated with an optical 1, and then using resulting data, calculated the laser diode's L-I slope to approximate the extinction ratio and provide some form of correction. See Bosch et al. U.S. Pat. No. 5,373,387, “Method for Controlling the Amplitude of an Optical Signal”, issued Dec. 13, 1994; and Bosch et al. U.S. Pat. No. 5,448,629, “Amplitude Detection Scheme for Optical Transmitter Control”, issued Sep. 5, 1995. Unfortunately, this extinction ratio determination method was inaccurate, as correction to the extinction ratio was sensitive to non-linearity (curvature) in the semiconductor laser's transfer function characteristics, and the method did not directly determine the actual extinction ratio.
As a consequence, a need remains for an apparatus and method to control the power levels and the extinction ratio of a semiconductor laser.